Metasomatic origin of diamonds in the world's largest diamondiferous eclogite

The world's largest diamondiferous eclogite (8.8 kg, UD-111/02) from the Udachnaya kimberlite, Siberia, is the subject of this investigation. High-resolution X-ray computed tomography (HRXCT), chemistry of host minerals and diamond inclusions, stratigraphy of the diamonds (cathodoluminescence zoning), and carbon isotopes have been obtained. HRXCT images, 3-d models, and subsequent detailed dissection of this Group B eclogite revealed linear distributions of diamonds along alteration zones. Both unusual multiple-diamond clusters with dodecahedral morphologies, and distinctive internal zoning recorded by cathodoluminescence analyses are indicative of multiple stages of diamond growth and resorption in UD-111-02. Carbon-isotope ratios (δ13C) of 16 diamonds have a total range of ~ 7‰ (− 2.0 to − 9.2‰), with the majority at − 5.4‰. Diamond core values are variable and include the lightest (− 9.2‰) and heaviest δ13C (− 2.0‰) values, suggesting that cores formed from distinct carbon sources, or grew under different conditions. In contrast, the rims of all diamonds have similar δ13C values (average − 5.2‰ with a 2SD of 1.2‰), suggesting that they formed under uniform conditions. Mineral inclusions in diamonds display compositional variations. These lines of evidence, considered together, indicate that diamonds in this eclogite were formed by several events, most likely from multiple metasomatic fluids. Furthermore, it is becoming increasingly evident that in most diamondiferous eclogites, diamonds are not synchronously formed with their host eclogites.